As is well known, the shortages of various materials in World War II, together with the natural development of the plastics industry over time, have led to the introduction in the several decades just past of a large number of new plastics, both thermosetting and thermoplastic in nature. Plastics have been used for a wide variety of purposes--for example, in the manufacture of automobile parts and bodies, toys, television cabinets, housewares, furniture, and many other products.
At the same time, as the use of thermoplastic materials has become more and more widespread, plastic welder devices have been developed to repair cracks, holes, tears and other defects that inevitably occur in these materials with use. Such devices have been available for many years, but all of them have suffered from one or more significant shortcomings.
Since thermoplastics break down chemically if they are exposed to temperatures substantially above their particular melting ranges, most plastic welders presently available require an air assist to keep the heating element of the welder from raising the temperature of the material being repaired to too high a level. Inclusion of air assist makes the plastic welder more complicated and bulky in construction, as well as more expensive to manufacture.
Means for reliably avoiding overheating of the plastic welder by intermittently interrupting the electrical current feeding the heating element in response to a temperature setting selected by the user (according to the type of thermoplastic materials being welded) have long been available, but for some reason no worker in this field prior to applicant appears to have believed that such expedients could be used to advantage in a tool of this type. This is true in spite of the increased economy of manufacture and efficiency of operation that are available through utilization of this approach.
Another substantial defect in prior art plastic welders is the difficulty of handling the tool during use. So far as applicant is aware, all plastic welders presently available require that the welder be held in one of the user's hands, while the supply rod of thermoplastic material for the weld to be made is fed into the welder with the other hand. Two-handed operation of the plastic welder as thus described is obviously awkward and inconvenient.
Still another shortcoming in prior art plastic welders is the susceptibility of their heating tips to damage from long continued use. In plastic welders known to applicant prior to his invention, the heating tips used to melt the objects to be welded and to melt the supply rod or filler rod are made of solid copper or of stainless steel. Such copper tips have not proved to be as durable as desirable, and both types have been quite expensive to fabricate.
The plastic welder of this invention avoids all these disadvantages of the prior art devices.